Aluminum-copper alloy



Patented Feb. 9, 1926.

UNITED STATES ZAY JEFFRIES, OF SHAKER HEIGHTS, AND ROBERT ASSIGNORS TO ALUMINUM COMPANY OF AMERICA, OF PITTSBURGH, PENNSYL-.-

1,572,487 PATENT orator-z,

S. ARCHER, 01? LAKEWOOD, OHIO,

VANIA, A GORTORATION OF PENNSYLVANIA.

ALUMINUM-COPPER ALLOY.

No Drawing. Original application filed January 4, 1921, Serial No. 435,024.

tion filed April 29, 1924. Serial No. 709,748.

To all whom it may concern Be it known that we, ZAY Jnrrnins, of Shaker Heights, and ROBERT S. ARCHER, of Lakewood, both in thecounty of Guyahoga and State of Ohio, have invented certain new and useful Improvements in Aluminum-Copper Alloys, of which the following is a full,-clear, and exact description.

The invention which forms the subject of our present application (a division of our copending application Serial No. 435,024, filed January 4, 1921 now Patent No. 1,508,556, issued September 16, 1924:) relates to the production of aluminum alloy castings, particularly castings made of alloys containing silicon or copper or both, with or without other metals, and its chief object is to provide a simple and effective method which will produce light-weight castings with a hitherto unobtainable combination of physical properties, especially as regards elastic limit, tensile strength and ductility. To this and other ends the invention consists in the novel method and product hereinafter described.

As will be seen from the subjoined description, the invention is based upon the combination of certain steps, some of them novel with us, and in part the invention resides in the discovery that certain alloys are especially susceptible to improvement by heat treatment after casting, and

that certain methods of casting such alloys furtherand peculiarly adapt them to such treatment; and in the additional discovery of temperature and duration of heat treatment in combinations appropriate to the particular alloys and methods of casting involved. l/Vith the aid of these discoveries we have been able to produce castingshaving improved physical properties with respect to elastic limit, tensile strength and ductility to an extent hitherto unattainable, with the result that it isnow possible to use aluminum alloy castings for purposes for which. their. use has previously been impracticable.

The tensile strength of commercially pure cast aluminum is about 12,000 pounds per square inch and its elongation about 25 per cent. By adding varying proportions of hardening metals, particularly copper Divided and this applicaand zinc, it is possible to increase the tensile strength so that the sand cast alloy will have an ultimate strength of nearly 30,000 pounds per square inch, but the elongation is thereby reduced to less than 5 per cent. By adding a very large amount of zinc the tensile strength may be raised to approximately 40,000 pounds per square inch, but the elongation is reduced to almost nothing, and the alloy is very brittle. By casting any of these alloys in a chill mold, the tensile strength may be increased in general by approximately 5,000 pounds per square inch and the elongation may also be increased. But it has heretofore been impossible to simultaneously produce tensile strengths of over 30,000 pounds per square inch and an elongation of 8 per cent or over.

By our method, however, we have been able to extend very greatly the range of the It is also to be noted that the high tensile .85

strength alloys previously known have required the presence of substantial amounts of zinc, even in many cases up to 33 per cent, and have therefore been considerably heavier than pure aluminum, while we obtain our results with an addition, in general, of not more than 5 to, 10 per cent of total heavier alloying metal or metals and yet produce a casting which is not only stronger and more ductile but likewise lighter than would otherwise be obtained. v Various prior investigators have proposed to obtain castings possessing the desirable combination of high tensile strength and ductility by heat treatment of a properly made casting, but their contributions to the art have so far not led to commercially useful results. For example, the best results published prior to our work are those ing its first step the preparation of an a of-Wilm, whoreports having obtained chill castings with a tensile strength of about 32,000 pounds persquare inch and anelontion of 5 to 7 per cent.' More recently rica and Karr have by heat treatment of an alloy containing copper and a large amount of zinc, obtained a tensile strength of 41,200 pounds per square inch but with an elongation of only 4 per cent. This alloy contained about 19 per cent of heavy alloygreater than 3. With lighter alloys, for example one containing small amountsof copper, magnesium and manganese, they obtamed a tensile strength of 35,500 pounds per square inch'and an elongation of 2.3 per cent. In none of the three cases mentioned above were the properties enou h better than those by untreated castings 'to 'ustify the cost of the treatment.-

invention involves the discovery of important reasons for rev-ious lack of ractical success along t is line, and emrades a combination of old and new steps so that we are now able by means of our improved method to produce art1cles,-cas t m either sand or chill molds, having phys1- cal properties far. superior to those of any such castin hitherto produced. By means of our nov method we have produced sand castings of an aluminum alloy containing 4 per cent copper and 0.2 per cent magnesium, having a tensile strength ofabout 50,000 pounds per square inch and an elongation of 8.5 per cent, with relatively high elastic limit; also chilled castings having a tensile strength of 54,000 pounds per square inch and an elongation of 1 8 per cent, with relatively high elastic limit.

This combination of high tensile strength and elongation enables these castings to be used for purposes for which aluminum castin have not hitherto been suitable and for w 'ich it has been necessary to use the much heavier ferrous materials such as steel or malleable iron.

One example of our process comprises as uminum alloycontamm' g) a out 3 to 5.5 per cent copper, prefera l 4 per cent, with no magnesium or wit magnesium up to about 0.3 per cent, the iron content in particular being as low as possible, preferably not to exceed 0.25 per cent. Ironand silicon are always present as im urities in aluminum as now obtainable, an a part of our lnvention consists in so choosing theraw materials .as to limit the iron content of the finished alloy to an amount as low as ossible, referably not over 0.25 per cent. he use 0? silicon in substantial amounts will be considered hereinafter.

The ingredients are mixed in the molten state, care being taken to avoid excessive temperatures at all stages of the melting metals and hence had .a specific gravity o oration, and the mixture is poured into either a sand mold or a chill mold, and caused to solidify. After solidification a microscopic examination reveals masses of an aluminum-rich constituent surrounded by a network of a hard, brittle constituent, which has been reported to be chiefly CuAl In the form of a sand-cast test bar about one half inch in diameter the alloy has a tensile strength of 18,000 to 25,000 pounds per square inch with an elongation of 2.5 to 4 per cent in two inches. A chill cast test bar of the same size has a tensile strength of about 24,000 to 28,000 pounds per square inch and an elongation up to 6 per cent in two inches. The iron in the alloy is found both in the form of needles, reported to be FeAl and in a different form, apparently as a silicide of iron. The presence of the iron (FeAl,) needles up to a certain amount is beneficial to the strength and ductility of the alloy in the cast condition, probably due to the fact that the ordinary ath of fracture ,in the cast alloys is throug the brittle network of CuAl which is strengthened by the iron needles. It is found that these iron needles are very detrimental to the physical properties of the casting after the eat treatment discussed below.

The next step in the example of our process now being described consists in heating the castings to, and maintaining them at, a temperature of 500 to 540 C. for a period of time depending upon the results desired and the manner in which the casting was made. The principal object of heating the castings at the temperature mentioned is to cause the CuAl in the network to go into solution in the solid aluminum-rich constituent. It is found that at the tem eratures referred to, this CuAl dissolves owly, in fact surprisingly slowly. In sand castings the time necessary for maximum solution of this CuAl may be as much as 48 hours, at the temperatures mentioned; while in chill castings, because of the finer network structure, the time for maximum solution is less-for example, 7 hours has been found to be suflicient in some cases.

If the co per content exceeds the amount which is so uble in the solid state just below the freezing oint of the eutectic, some of the CuAl wi 1 remain in the network after heat treatment, andthe physical properties of the castin will not be as good as can be obtained wit a lower copper content. In

most if not all cases the best combination of physical properties results only when substantlally all of the CuAl in the network has been dissolved in the aluminum-rich is comparatively long.

A'further result of heatin "the castings at the tem erature mentioned is to re-dissolve any. tated in small particles within the aluminum-rich constituent rather than in the network during the previous cooling of the castings.

After the heating period the castings are preferably cooled rapidly, asby quenching in water. By the above described procedure, sand castings of an alloy containing 4 per cent copper, 0.2 per cent magnesium and less than .25 per cent iron have been produced with a tensile strength, after aging, of about 50,000 pounds per'square inch and an elongation of 8.5 per cent. Chill castings have been made by the same process, of the same alloy, havinga tensile strength of 54,000 pounds per square inch after aging and an elongation of 18 per cent.

As stated above, quenching is preferable to slow cooling, but good results canin some cases be obtained by cooling in a current of air, A marked difference has been observed between the results obtained by cooling in a current of'air and those obtained by cooling in still air. Even slow cooling, however, produces improvement as compared to the cast condition, not only in the physical properties but also in resistance to corrosion.

The'physical properties of castings subjected to the treatments above referred to can be further changed by artificial aging, that is, by heating them, immediately after cool ing, to'a temperature of 100 to 150 C. for a time, one hour being in many cases sufiicient. As a result of such heating the tensile strength is increased and the elongation decreased, and the elastic limit is increased very markedly. V The method of casting is an important factor. Thus'a method producing fine grain and improved physical properties, yields castings which are generally more amenable to the heat-treatment than does a casting method which produces coarse grain. For example a bar having a cylindrical test section two inches long and a ha1f-inch in diameter, composed of alloycontaining about 4 per cent copper and cast in. a sand mold, has a tensile strength of about 18,000 pounds per square inch and an elongation of about 4 per cent. If the bar is heat-treated by our method, say at a temperature around 520 C. for 24'hours, the tensile strength maybe more than doubled (increasing to about 37,000 pounds per square inch) and the elongation is increased to about 12 per cent, or three times its original value- On uAl which may have precipi-.

the other hand, if the bar is cast in such a manner as to cause rapid solidification, say by chill casting in an iron-mold, it will have, after heat-treatment by our method, a tensile strength of about 40,000 pounds per square inch, and an elongation of about 20' per cent.

The addition, to the aluminum-copper alloys, of a small amount of magnesium, say 0.2 or 0.3 per cent, increases the tensile strength and elastic limit, but is less favor-- able to ductility or elongation.

For the best results, our experience indicates that the iron content of the aluminum copper alloy should be low, but if care'is taken to make the castings by the chill method, a higher percentage of iron can be used, good results being obtained with chill castings containing even more than 0.4 per cent of iron. On the other hand, in the case of heat-treated sand castings, more than that amount of iron gives less advantageous physical properties. The presence of the iron is evidenced by crystalline needles, probably of the composition FeAl,, which usually form in the net 'work containing CuAh. These needles are not soluble to any considerable extent in the solid aluminum alloy, and remain intact and in place while the CuAl is absorbed into the body of the solid-solution crystals of aluminum during heat-treatment. The iron needles tend to reduction below about 0.25 per cent is not always practicable commercially. However, when silicon is present iii-excess of the iron, the quantity of FeAl, formed islessened, and some of the iron combines with the silicon to form what appears to be an iron sili-. clde. We have discovered that the latter compound freezes in a form less harmful than the needles of FeAl In the ease of chill castings, it is found that the FeAl, needles are much smaller'than in the sand castings, and hence they are not so objectionable in the heat-treated casting. Since there is considerable difiiculty in commercially produeingaluminum free from or containing less than 0.25 per cent of iron, the desired silicon-iron relationship is most advantageously obtained by adding silicon when necessary. For example, if the. iron content of the aluminum ingot is 0.35 per cent and the silicon 0.3 per cent, we find it advantageous to add about035 per cent silicon. lVe have also found that the addithat beyond 5.5 per cent copper some of the CuAl usually remains out of solution even after heat-treatment. In some cases the metal can be heated above the melting point of the eutectic of CuAl and aluminum without impairing its physical properties provided the metal is cooled to a temperature slightly below that of complete solidification and held there for some time before quenching. When substantial percentages of iron, silicon, magnesium and zinc are present, or any of them, the higher permissible temperature is lowered.

It is found that the coarser the grain produced in casting, the longer must the heattreatment be continued to roduce the most beneficial results. Accordingly sand castings with large cross-sectional areas require longer heating than chill castings. Usually holding at temperature for about 7 hours is sufficient for chill castings, whereas 24 hours may be necessary with sand castings.

In addition to the production of a superior grade of sand castings, permanent mold or chill castings (including die castings), the application of our invention is also most im-' portant in connection with the forging and pressing of aluminum alloys. If it is desired to produce a forging of a given shape, a casting of suitable composition, say 4 per cent copper and 0.2 per cent magnesium, is made, preferably in a chill mold, and is subjected to heat-treatment by our method to produce a starting material for forging.

In this case, instead of quenching from the heat-treatment temperature and then reheating the article to forging temperature, we find it preferable to simply cool slowly from the heat-treatment temperature to the forging temperature. The piece is then finished to size under the press or hammer, after which it is given a short heating, say at 520 0., followed by quenching.

The duration ofthe heat-treatment step of our method depends, in large measure, other conditions being the same, upon the degree of improvement desired in the physical properties of the casting.

The essential thing to be kept in mind is the relatively long time required for east alloys in general, as compared to those rolled aluminum-copper alloys known to the prior heat-treating art. This is caused, in the case of the cast copper alloys, by the greater diflicult'y in' getting the unbroken CuAl network into solution in the aluminum.

In the copper alloys containing about 4 per cent copper, for example, it is found that heating for very long periods, say 48 hours or more, at temperatures only slightly below 500 C. will not, in general, produce as good results as a five hour treatment at 520 C. In chill castings of the copper alloys it has been found that even two hours heating at 520 C. produced substantially as good results as about 22 hours at 500 C. In order to reduce the time to'a minimum, we prefer, if suitable temperature control is available, to use as high a temperature as can be safely employed without spoiling the castings by overheating.

It is to be understood that the invention is not limited to the specific details-herein described but can be practised in other ways without departure from its spirit.

We claim- 1. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting.

2. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper, casting the alloy, heating the casting to a'temperature of about 500 to 540 C. for atleast about 7 hours, and coolingthe, casting rapidly.

3 In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy low in iron and containing between about 3 and 5.5 per cent of copper, casting the alloy, heating the casting to a temperature of about 500 to' 540 C. for at least about 7 hours, and cooling the casting.

4. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy low in iron and containing between about 3 and 5.5 per cent of copper, casting the alloy, heating the casting to a temperature ofabout 500 to 540 C. for at' least about 7 hours, and cooling the casting rapidly.

5. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of'copper and not more than about 0.3 per cent of magnesium, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting.

6. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent' of co per and not more than about 0.3 per cent of magnesium,

casting the alloy, heating the casting to a. temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting ra idly.

In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy low in iron and containing between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent of magnesium, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the castin 8. In the art of ma -king aluminum-alloy castings, the method comprising reparing an aluminum alloy low in iron an containing between about 3 and 5.5 per cent of copper and not more than about 0.3. per cent of magnesium, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting rapidly.

9. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about '3 and 5.5 per cent of copper and a small amount of zinc, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting.

10. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, cooling the casting, and reheating the alloy at a relatively low temperature for a time sufiicient to increase the tensile strength of the alloy.

11. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per. cent of copper, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 Hours, cooling the casting rapidly, and reheating the alloy at a relatively low temperature for a time sutficient to increase the tensile strength of the alloy.

12. In the art of making aluminum-alloy castings the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent of magnesium, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, cooling the casting, and re-heating the alloy at a relatively low temperature for a time sufficient to increase the tensile strength of the alloy.

13. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy low in iron and containing between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent of magnesium, casting the alloy, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours,-cooling the casting, and reheatin the alloy at a relatively low temperature %or a time suflicient to increase the tensile strength of the alloy.

14. As a. new article of manufacture, a heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, characterized by the substantial absence of undissolved inter-granular copper-rich constituent, and having high tensile strength and elongation.

15. As a new article of manufacture, a heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper and a relativel small amount of magnesium, characterize by the substantial absence of undissolved inter-granular copper-rich constituent, and having high tensile strength and elongation.

16. As a new article of manufacture, a heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, and magnesium not exceeding about 0.3 per cent, characterized by the substantial absence of undissolved inter-granular copper-rich constituent, and having high tensile strength and elongation.

17. As a new article of manufacture, a heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper and having a low iron content,

characterized by the substantial absence of undissolved inter-granular copper-rich constituent, and having high tensile strength and elongation.

18. As a new article of manufacture, a heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper and a relatively small amount of magnesium, and having a low iron content, characterized by the substantial absence of undissolved inter-granular copper-rich constituent, and having high tensile strength and elongation.

19. As a new article of manufacture, a heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, and magnesium not exceeding about 0.3 per cent, and having a low iron content, characterized by the substantial absence of undissolved inter-granular copperrich constituent, and having high tensile strength and elongation.

20. As a new article of manufacture, a heat-treated, artificially aged casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, characterized by high tensile strength and substantial absence of undissolved inter-granular copper-rich constituent. 7

21. As a new article of manufacture, a heat-treated, artificially aged casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper and a relatively small amount of magnesium, characterized by high tensile strength and substantial absence of undissolved inter-granular copperrich constituent.

22. As a new article of manufacture, a heat-treated, artificially aged casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, and magneslum not exceeding about 0.3 per cent, characterized by high tensile strength and substantial absence of undissolved inter-granular copper-rieh constituent.

23. In the art of making aluminum alloy castings, the method comprising preparing an aluminum alloy containing between about 3 to 5.5 per cent of copper, casting the allo and causing rapid solidification thereo heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting.

24. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper, casting the alloy and causing rapid solidification thereof, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting rapidly.

25. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent 0 magnesium, casting the alloy and causing rapid solidification thereof, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting.

26. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent of magnesium, casting the alloy and causing rapid solidification thereof, heating the casting to a' temperature of about 500 to 540 C. for at least about 7 hours, and cooling the casting rapidly.

27. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper, casting the alloy and causing rapid solidification thereof, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, cooling the casting, and reheating the alloy at a relatively low temperature for a time suflicient to increase the tensile strength of the alloy.

28. In the art ofmaking aluminum-alloy castings, the method comprising preparing an aluminum alloy containing between about 3 and 5.5 per cent of copper, casting the allo and causing rapid solidification thereof, heating the casting to a tempera ture of about 500 to. 540 C. for at least about 7 hours, cooling the casting rapidly, and reheating the alloy at a relatively low temperature or a time sufiicient to increase the tensile strength of the alloy.

29. In the art of making aluminum-alloy castings, the method comprising preparing an aluminum alloy contaimng between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent of magnesium, casting the alloy and causing ra id solidification thereof, heati the castmg to a :temperature of about 500 to 540 C. for at least about 7 hours, cooling the casting, and reheating the alloy at a relativel low temperature for a time sufiicient to increase the tensile strength of the alloy. 1

30. In the art of making aluminum-alloy castings, the method comprising reparing an aluminum alloy low in iron an containing between about 3 and 5.5 per cent of copper and not more than about 0.3 per cent of magnesium, casting the alloy and causing rapid solidification thereof, heating the casting to a temperature of about 500 to 540 C. for at least about 7 hours, cooling the casting, and reheating the alloy at a relatively low temlperature or a time sufficient to increase t e tensile, strength of the alloy.

31. As a new article of manufacture, a chill-cast, heat-treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, characterized by the substantial absence of undissolved intergranular copper-rich constituent, and having high tensile strength and elongation.

32. As a new article of manufacture, a chill-cast, heat treated casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper and a relatively small amount of magnesium, characterized by the substantial absence of undissolved intergranular copper-rich constituent, and having hi h tensile stren th and elongation.

33. is a new article of manufacture, a chill-cast, heat-treated and artificially aged casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper, characterized by high tensile strength and substantial absence of undissolved intergranular copper-rich constituent.

34. As a new article of manufacture, a chill-cast, heat-treated and artificially aged casting of an aluminum alloy containing between about 3 and 5.5 per cent of copper and a relatively small amount of magnesium, characterized by high tensile strength and substantial absence of undissolved intergranular copper-rich constituent.

In testimony whereof we hereto aflix our signatures.

ZAY JEFFRIES. ROBERT S. ARCHER. 

